1. Field of the Invention
The invention relates to a brake valve for motor vehicles and, in particular, relates to a control circuit or a brake valve pressure feedback circuit for regulating the pressure in a plurality of brake cylinders.
Such brake valves are, for example, employed in motor vehicles within single-circuit or twin-circuit power-brake systems in order to ensure a sensitive build-up and reduction of the brake pressures present at the motor vehicle brake cylinders on actuation of the valve. These brake valves are preferably fastened in this case to the floor panel of the motor vehicle so that they may be conveniently and directly actuated by means of a pedal connected to the brake valve.
2. Description of the Prior Art
Known brake valves of this generic type, with the principle of their connection and examples of hydraulic switching arrangements using these brake valves, are represented as examples in FIG. 5 through FIG. 9.
The known brake valves 10 shown in FIG. 5(A) and 5(B), in single-circuit embodiment in FIG. 5(A) and twin-circuit embodiment in FIG. 5(B), are directly controlled pressure reducing valves of three-way embodiment with brake circuit pressure protection and stepless mechanical actuation, i.e., with the pressure in the brake circuit steplessly adjustable in proportion to the given actuation force.
These valves 10 are acted on by a hydraulic pressure via passages S.sub.P from brake system hydraulic reservoirs 30 and a pump (not represented), and are connected, via passages B.sub.R to brake conduits 12. Brake conduits 12 lead to the brake cylinders 20 of the motor vehicle and are connected to a receiver or tank 40 via passages T. Furthermore, the brake pressure is fed back within valve 10, with or without throttling, via a control pressure hole or conduit Z for the closed-loop control of valve 10.
As an example for both brake valve embodiments, the mode of operation is described below using single-circuit embodiment of FIG. 5(A).
The brake valves 10 are respectively actuated by means of an actuation element 16, such as a pedal. This actuation element 16 presses a main compression spring 17A against a control plunger 13 of valve 10. A control edge (not represented) of the control plunger 13 closes passage T and the through-flow from passage S.sub.P to passage B.sub.R is then released. The pressure which now builds up in brake conduit 12 also acts simultaneously, via the control fluid hole Z, in valve 10 behind control plunger 13 and acts against main compression spring 17A so that the brake pressure increases in proportion to the actuation force on spring 17A. If the actuation force is kept constant, control plunger 13 goes into the closed-loop control position and therefore keeps the adjusted value in passage B.sub.R, substantially constant until there are control deviations. If main compression spring 17A is relieved, a compression spring 17C closes the passage between S.sub.P and B.sub.R and opens the passage between B.sub.R and T by means of control plunger 13 so that the brake circuit is relieved.
The mode of operation of the twin-circuit embodiment of the valve 10 in FIG. 5(B) is given, in principle, by the parallel connection to the pump of two single-circuit embodiments, control plungers 13 being connected in series relative to one another by springs 17A, 17B, and 17C. Furthermore, a distance piece or stop piece 17D is arranged between control plungers 13. On actuation of actuation element 16, the actuation force is transmitted to second control plunger 13.2, via first control plunger 13.1, by means of distance or stop piece 17D. In the closed-loop control position of control plungers 13.1 and 13.2, these plungers are elastically loaded relative to one another by means of a spring 17B fitted between them so that a relative motion between two control plungers 13.1 and 13.2 for individual adjustment of the pressure in the respective brake circuit is made possible. At control fluid holes Z, connected to the respective brake circuits in valve 10, main compression spring 17A and spring 17B connected between plungers 13.1 and 13.2 act against each other, oppositely in this case also, so that the brake pressure increases in proportion to the actuation force. These control fluid holes Z may have a throttle or orifice 18 in valve 10. Furthermore, control fluid hole Z, located in the brake valve 10, for first control plunger 13.1 located nearest to actuation element 16 and may be additionally connected from actuation direction to one control end of second control plunger 13.2. This ensures that both brake circuits have approximately the same pressure level by means of follow-up control.
FIGS. 6 and 7 show typical circuit configurations of brake valves 10 of the generic type, a single-circuit power-brake system being represented in FIG. 6 and twin-circuit power-brake system being in FIG. 7, with a handbrake 70 in each case. In these circuits, power-brake valve 10 acts directly, in a known manner, on wheel brake cylinders 20 via brake conduits 12. Additionally, the brake pressure being measured in valve 10 and control plunger 13 of valve 10 is adjusted in proportion to the measured pressure or control pressure.
In the power-brake valves of the generic type, it has been found that the buildup times for brake pressure in brake conduits 12 are very long, particularly in the case of long brake conduits 12. This becomes noticeable, particularly when low brake pressures are desired. This is because the pressure build-up occurs more slowly at a lower pressure value that at a high pressure value. In addition, the response or feel of the brake for the person actuating the brake is greatly impaired in these cases. This increases the danger of over-braking.
In order to obviate these disadvantages, a hydraulic brake system has therefore been proposed such as is realized, for example, in the brake systems of FIGS. 8 and 9.
FIGS. 8 and 9 represent power-brake systems of twin-circuit configuration with an additional third handbrake circuit 70. These brake systems are employed in very large motor vehicles, for example, heavy transport vehicles, where the lengths of the brake conduits assume substantial lengths. For this reason, relay valves 80 with hydraulic reservoirs 30 are provided directly at each brake axle in these known brake systems. The supply to these brake systems is ensured by a pump P and a plurality of hydraulic reservoirs 30 which respectively act with hydraulic pressure on relay valve 80 of the first or second brake circuit. A twin-circuit power-brake valve 10 of the generic type is, in addition, intermediately connected in the secondary circuit (control circuit) of the relay valves 80 so that it may only be used for triggering relay valves 80. This arrangement ensures short response times for the brakes even in the case of long brake conduits.
Specific disadvantages of the brake systems of this type are, however, the high level of structural complexity, the high weight and the sppace requirements necessary; these are mainly due to the need for hydraulic reservoirs at the axle. In addition, such power-brake systems with relay valves 80 are very cost-intensive due to the number of additional components required and for the reasons quoted above.